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The transients of volume-controlled doubly injected currents and of the accompanying recombination radiation were measured on anthracene crystals. By this method, which allows the trap-free case to be approximated, the carrier recombination rate constant K was determined in a more reliable way than is possible with steady-state currents. The result, K=(1.05±0.5)×10−6 cm3 sec−1, agrees well with the theoretical value predicted by a model in which the rate is a function of carrier mobility. The recombination fluorescence was found to consist of two components differing by their rise times. The fast component, Ifast, is due to singlet excitons generated directly by carrier recombination. The slow component, Islow, is proof that carrier recombination produces also triplet excitons (which can recombine to give singlets producing fluorescence). The ratio Islow/Ifast=0.65±0.1 indicates that two recombining triplet excitons do not always produce a singlet exciton but may also form an excited triplet exciton. In addition, the light transient measurements permitted the determination of the total rate constant, γtotal, of triplet—triplet recombination in a novel way. The result is γtotal=(4–8)×10−12 cm3 sec−1.
Helfrich et al. (Fri,) studied this question.